1. Field of the Invention
This invention relates to ion implantation into a semiconductor substrate, and more particularly to boron ion implantation into a semiconductor substrate.
2. Description of the Related Art
Doping impurities of a desired conductivity type into a desired region is an important technology for manufacturing a semiconductor device structure. Ion implantation is widely used as the impurity doping method. In ion implantation, ions of a desired species are given acceleration energy, and implanted into a semiconductor substrate. Then, the implanted impurity atoms are activated by annealing to form electrically active impurity doped region. The depth and the resistivity of the impurity doped region are controlled by selecting the ion acceleration energy and the dose. Usually, boron ions are implanted to form an a p-type region in a silicon substrate.
Miniaturization of MOS transistors in size is required to realize increases in operation speed and integration density of a semiconductor device. When a MOS transistor is scaled down according to the scaling low, the junction depth for source/drain regions of a MOS transistor having a gate length of 0.1 .mu.m becomes around 0.05 .mu.m. The junction depth of this order is difficult to be realized by the conventional ion implantation system.
In the general ion implantation system, the beam current becomes smaller as the acceleration energy becomes smaller. The ion beam has a tendency of expanding the beam diameter due to the beam potential which the ion beam has by itself. It is known that the expansion of the beam diameter is proportional to the beam current and inversely proportional to 1.5 powers of the energy. If the acceleration energy is lowered to form a shallow junction, the efficiency of the beam becomes worse and the beam current will become low.
For forming a MOS transistor having gate length of 0.1 .mu.m or less, the acceleration energy for boron ions should be 5 keV or less. When boron ions are implanted at an acceleration energy of 5 keV, the electric current of the ion beam in a conventional ion implantation system becomes 1 mA or less. If MOS transistors are manufactured using such a low ion beam current, the productivity will significantly be lowered and the manufacturing cost will increase.
For implanting ions shallowly, the mass of ions to be implanted may be increased, as well as lowering the acceleration energy. For implanting boron ions, a method of using BF.sub.2 ions is proposed. Compared to the ion implantation of single boron atoms, ion implantation of BF.sub.2 ions will decrease the effective acceleration energy for each single boron ion to about 1/5. Therefore, implantation of BF.sub.2 is advantageous to form a shallow junction.
When a MOS transistor is manufactured by using BF.sub.2 ion implantation, however, it is known to cause another problem. Ion implantation to the source/drain regions of an MOS transistor will usually implant ions also to the gate electrode. A silicon gate electrode should be doped with a sufficient amount of impurity, for preventing depletion of the gate electrode. In a minute MOS transistor, the gate oxide film should also be formed thin, for example, 10 nm or less. When BF.sub.2 ions are used as a source of B atoms, fluorine atoms are also implanted and will accelerate the diffusion of boron atoms through the gate oxide film. When boron atoms pass through the gate oxide film due to the influence of fluorine atoms, and diffuse into the channel region under the gate oxide film, the threshold voltage of the MOS transistor will be varied. Therefore, BF.sub.2 ion implantation cannot be easily employed in a p-channel MOS transistor having a thin gate oxide film.
Also, in case of implanting ions into a semiconductor device having minute patterns, charging phenomenon may become problem. When the gate electrode is charged with positive charges and these positive charges discharge through the gate oxide film, the gate oxide film will be deteriorated. The deterioration becomes more significant as the ion current flowing through the gate oxide film becomes larger.